The invention pertains to an integrable circuit for identifying a pilot tone from a number of pilot tones contained in an audio-frequency signal by means of pilot-tone filters tuned to the individual pilot tones.
Such a pilot tone is used, for example, to identify the stereo signal of a stereo broadcasting system, as is described in European Pat. No. 0 047 522. Since different stereo broadcasting systems are currently in use, it is at least desirable to have an automatic changeover facility at the receiving end which ensures that the stereo signal is decoded in a suitable manner even if there is a change to a transmitter of a different stereo system.
In the U.S.A., for example, different stereo-broadcast techniques are currently in use. In these techniques, the sum signal (L+R) is transmitted using amplitude modulation because compatibility with existing AM mono receivers is required. The difference signal (L-R) is transmitted using frequency, phase, or quadrature modulation. A survey of these techniques can be found, for example, in the Apr. 26, 1982 issue of the "Electronic Engineering Times" on pages 18 and 20. According to that publication, an integrated AM stereo decoder is already obtainable.
To identify the signal of a given stereo-broadcasting system, a low-frequency pilot tone that is characteristic of the system is transmitted with the carrier. It is filtered out and automatically turns on the receiver's stereo decoder. Such receivers are described in U.S. Pat. Nos. 4,302,626, 4,232,189 and 4,018,994. In the first-mentioned patent, it is stated that a pilot tone can be extracted by means of a digital filter. The invention uses exclusively digital filters which are generally known.